240.21 Fuse Question

[Anderman123]

I have a 24VDC Power supply (powered with 120VAC). I am using it for control circuits. The 0VDC leg is ungrounded. My co-workers insist that both the +24VDC and the 0VDC from the power supply need to be fused. I am asuming they are reading 240.21, but I read that as only needing the fuse on "where the conductors receive their supply" or the +24VDC.

Any advice?

 

[nhfire77]

Is this a Class 1 or 2 circuit?

Either way I don't believe you need to fuse both legs, did your brain trust coworkers give their reasoning?

 

[Anderman123]

This will be a class 2 circuit.

Their reasoning seems to be that 240.21 says that ungrounded circuit conductors must be fused.....that is as as much as I can figure.

 

[infinity]

Class 2 circuits are power limited. They generally do not require an OCPD on the load side of the transformer. They do need to comply with 725.127 for the OCPD on the line side. I would read through Article 725 for more information.

 

[nhfire77]

I cannot imagine a technical reason for it, and I disagree with their interpretation.
I have installed 10's of thousands of class 2 circuits, 1000's under engineer supervision/drawings, never once have I seen both fused, and rarely, if ever one; if the power supply have OL protection such as a PTC. Now distributing circuits, I would fuse them individually, such as analog CCTV 24 VAC circuits.

Certain applications do need fusing and is good practice.

What is this for anyway?